I'd like to preface this article by saying there are lots of important principles to packing on size and all must be followed. As you become a more advanced lifter you must follow the principles more strictly and parts of your training become more nuanced. As a beginner you can almost get away with doing anything and you'll grow thanks to those good old newbie gains, however there's one principle which simultaneously becomes ever so more important, and for most becomes overlooked until you take step back and really analyse why you aren't growing anymore.
If you're reading this, you probably know the basics as to what goes into hypertrophy:
In this article I'll be focusing on rest, which indirectly effects your training. Here you can take a deep dive into how sleep aids in muscle recovery.
Training at a high enough intensity is crucial during your training. How much intensity is optimal, is a much debated topic with new research emerging constantly on what is optimal. This will of course differ from lesson to person and what you're specifically training for. However, one thing is clear, you need to be close to failure, and research suggests that it doesn't really matter at what rep range that will be, in a pure muscle building context - remember that strength and size are not necessarily the same thing.
So what does this have to do with rest? Well, if you're fatigued you aren't going to be able to train at your optimal intensity - you simply won't be able to overload your muscles as much as they could be, which means a sub optimal stimulus for growth.
CNS, or Central Nervous System Fatigue is fairly complicated principle, but in short, it's your body's way of protecting itself from over exertion. Your body will literally limit the performance of itself or a certain muscle group by means of limiting motor-neuron recruitment.
Muscle fatigue simply means the fatigued muscle hasn't finished repairing itself from your previous workout. For optimal results, you'd want to wait until your muscle has completely recovered then immediately train it to repeat the never ending cycle. You should note that different muscles have different recovery times, which of course also depends on the intensity and volume of your previous workout.
As you lift heavier and heavier the amount of stress and motor-neuron recruitment gets greater and greater, leading to a demand for more and more rest. In this context, less is definitely more.
So if you're fatigued, your muscles are working at a reduced output efficiency, thus you're really going to struggle to consistently overload week in, week out... and so you platue. Thus, you're not giving your muscles the stimulus of moving a heavier weight than what it's used to, and so it won't grow.
You're probably thinking this is obvious, but let me ask you something:
Have you ever taken a full week off, say from a holiday, returned to the gym and instantly hit a new PR - you feel incredibly strong don't right?
Then you probably return to your push pull legs, or bro split, make progress for a week or two, then it slows down.
You need to program better. Take more rest, lower the frequency of the super demanding compounds, and if you're a body builder, you replace certain movements with more stabilised, less taxing movements which give you a greater stimulus to fatigue ratio.
And thar my friends, is in my humble opinion, the most overlooked factor in building muscle.